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FALL 2003

GEOLOGY 284:

MINERALOGY

Dr. Helen Lang

Dept. of Geology & Geography

West Virginia University

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Mineral Propertiesin Hand Specimen

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Luster

Metallic Sub-metallic

Non-metallic Vitreous

Adamantine

Resinous Greasy

Silky Pearly

Dull

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Adamantineto sparkle and appear

brilliant like diamond

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Diaphaneity(ability to transmit light)

Transparent

Translucent

Opaque

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Color

Chromophores

Elements that give minerals color

Mainly transition metals (Fe, Ti, Cr, Mn, etc.) Idiochromatic (self-coloring)

Major or necessary elements determine color More likely to be useful for identification

Allochromatic (other-coloring)

Minor or trace elements determine color

Ruby red is from trace Cr in corundum (Al2O3)

Sapphire blue is from trace Fe and Ti in Al2O3

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Staurolite is

idiochromatic Fe gives staurolite its

brown color Staurolite formula is

Fe2Al9Si4O23(OH)

Mn gives rhodochrosite

its cherry red color Rhodochrosite formula is

MnCO3

Rhodochrosite is

idiochromatic

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Structural defects may

give quartz color

Radiation damagecauses purple

(amethyst) or smoky

color

trace Fe givesaquamarine its color

Aquamarine is Beryl,

formula Be3Al2Si6O18

Aquamarine is

allochromatic

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Streakis a useful diagnosticproperty

Color of fine powder on streak plate

More reliable than hand-specimen color

Especially useful for dark, metallic minerals

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Play of Colors

Opalescence - whitelight is separated into

many colors (opal

and moonstone)

Chatoyancy & asterism- scattering effects

Opal

Star Sapphire

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Crystal Shape

Special named shapes

cube Pyrite

dodecahedron Garnet

octahedron Magnetite

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Crystal Shape

General Shapes

prismatic

platy

blocky tabular

bladed

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Crystal Habit (in aggregates)

massive

granular radiating

acicular

fibrous

asbestiform

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Crystal Habit (in aggregates)

colloform orglobular

botryoidal(bubble-like)

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Cleavage

Shiny, smooth,

planar breakage

Between weakly

bonded planes in

mineral structure

Sometimes hard to

tell from growthfaces (commonlyhave imperfections,

not as smooth)

By quality

perfect

good

fair

By shape or

number

cubic rhombohedral

octahedral prismatic

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Cleavage

Examples

Fluorite: perfect octahedralcleavage (4 directions, 8 sides)

Note difference from Fluorite

growth faces (cubes)

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More Cleavage

Examples

Note mirror-like reflections

off Muscovite basal cleavages

Hornblende Cleavages in thin

section: note 56o

and 124o

angles

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Hardness (relative)

Mohs Hardness Scale

talc H=1

gypsum H=2

fingernail H=2.5

calcite H=3

penny H=3.5

fluorite H=4

apatite H=5

knife, glass H=5.5

feldspar H=6

file, streak plate H=6.5 quartz H=7

topaz H=8

corundum H=9

diamond H=10

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Density (or Specific Gravity)

Density () is in grams/cubic centimeter (g/cc) Specific Gravity (G) is mass mineral / mass

water at 1atm and 4o

C (~1; therefore, and Gvalues are similar; G is unitless)

Determined by weight in water vs. weight in air

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Density (or Specific Gravity)

Relative density (or heft) is useful for

identifying some minerals

most minerals range from G=2 to G=8 average G non-metallic minerals ~2.7 (quartz)

average G metallic minerals ~5 (pyrite)

Barite (BaSO4) with G=4.5 feels heavierthan

most other light colored, translucent minerals

(of the same size)

Galena (PbS) with G=7.6 feels heavierthan

most other metallic minerals (of the same size)

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Other Useful Properties

A few minerals are magnetic (attract a magnet)Magnetite and Pyrrhotite

Some carbonate minerals effervesce (fizz) in

dilute (5%) HCl: Calcite, Rhodochrosite,

Aragonite; others, especially Dolomite, do not

Fluorescence, phosphorescence,thermoluminescence orradioactivity are

diagnostic for a few minerals